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\f0\b\fs24 \cf0 From the Mississippi to the Ganges, River Deltas Are in Major Trouble
\b0 \
by Chris Mooney\
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\cf0 Aug. 6, 2015 \'96 A river delta is, by definition, a place in flux. Coastal land naturally sinks, but is naturally rebuilt by the flow of a vast river that carries in new sediment. Across the globe, from the Amazon to the Nile to the Yangtze, we humans rely on such deltas for the many benefits they bring \'97 access to fisheries, good locations for shipping, and much else.\
But we don\'92t just rely on them \'97 we change them.\'a0 We\'a0dam rivers upstream and channelize them downstream \'96 actions that reduce the flow of sediment and, thus, the growth of land. Meanwhile, we cut channels through wetlands and cause land to sink further by pulling lots of oil and gas and water out of\'a0it.\
Such changes have an insidious consequence: even as they entice larger populations to live on deltas, by providing jobs and the semblance of environmental stability, they also increase long-term vulnerability to storms and floods, by exacerbating the land\'92s sinking. Meanwhile, global warming makes the whole dynamic worse, because it introduces a huge arrow pointing in\'a0only one direction \'97 sea level is going up, and up, and up.\
Now, a sweeping new\'a0study\'a0released in
\i Science
\i0 Thursday takes a comprehensive look at\'a0how much such factors are increasing the vulnerability of 48 major global river deltas, the home to 340 million people \'97 including the Mississippi River delta, which lies between the city of New Orleans and the sea.\
\'93We characterized the rate of change of risk in delta systems, due to combined land subsidence, sea level rise, geophysical setting, and socio-economic capacity to protect themselves,\'94 explains\'a0lead study author Zachary Tessler of the City University, who conducted the research with scholars from 3 additional universities.\
The study\'a0used\'a0multiple datasets and indices to\'a0examine changing risks to river deltas, based upon 1) their inherent risk of storms and floods; and 2) how much humans are exacerbating those risks, by changing their landscapes in a way that causes land to sink, even as seas rise.\
Along the 1st axis, deltas range from the Orinoco in South America, which faces relatively few flooding and storm threats, to the Yangtzee and Han, where these risks are very high.\
On the 2nd axis, when it comes to human perturbations of the system, meanwhile, river deltas range from the pristine Yukon in Alaska, which is relatively lowly populated and has been little changed by humans in a way that would make it more vulnerable (much of it is a wildlife refuge), to the Ganges-Brahmaputra of Bangladesh and India \'97 home to over 100 million people, where human activities and sea level rise are pushing the risks forward dramatically.\
Based on these\'a02 factors \'97 how much human activities and sea level rise are changing a\'a0delta and worsening its plight, and how much it is exposed to storm\'a0and flood\'a0risks \'97 the study found a complex tapestry of changing risks, depending on the delta. On the one hand, the Yukon wasn\'92t at much additional risk at all. \'93It\'92s definitely clear that some of these deltas are almost completely untouched, that tends to be at the high latitudes,\'94 says Tessler.\
However, most\'a0of the deltas showed at least some increased risk, and some showed quite a lot of it. In particular, the heavily populated Krishna and Ganges-Brahmaputra deltas had the most rapidly growing risk of flooding related disasters. \'93They have\'85lots of dams, lots of development on the coastline, wetland loss \'97 we expect the relative sea level rise that\'92s happening there to have a large impact on the risk outcomes of the communities,\'94 Tessler says.\
Indeed, Bangladesh has historically been the home to the deadliest tropical cyclone (or hurricane) disasters known to history, including the 1970 Bhola cyclone, which struck this very delta, the Ganges-Brahmaputra. It caused 300,000 to 500,000 deaths.\
But there\'92s another key factor involved in determining a given delta\'92s risks \'97 a society\'92s wealth, which translates into its ability to protect itself. Thus, while the Mississippi River is heavily managed and has been changed nearly beyond recognition by humans, the delta also has a high level of artificially imposed resilience \'97 with vast levees along much of the river to prevent flooding, not to mention New Orleans\'92 huge\'a0new hurricane protections.\
A similar story can be told about the Rhine, heavily altered by humans but also the home to arguably the world\'92s most impressive flood protections, constructed by the\'a0Dutch. \'93A high capacity for investment in risk-reducing technologies \'85 is the primary reason that several wealthy, developed deltas today have relatively low risks,\'94 the study found.\
But that may not always be the case. The research\'a0found that if the cost of building flood-defense infrastructure increases steadily over 100 years \'97 as might be brought on, for instance, by rising energy prices \'97 then the rich but heavily exploited deltas, like the Mississippi, would see a spike\'a0in vulnerability. In that scenario, a delta like the Mississippi \'93reverted back toward expectations, based on geophysical hazards and anthropogenic change alone.\'94\
That\'92s a\'a0sobering message this year, as most observers of New Orleans 10 years after Hurricane Katrina are likely to look at its gigantic and costly hurricane protections, and see them as a sign of reassurance. They are \'97 but Tessler is suggesting that it\'92s a little more complicated than that.\
\'93If you want to maintain your [level of] risk, in light of increasing cost, you need to spend increasing amounts of money,\'94 he says. \'93And while you\'92re doing that, you\'92re not addressing the fact that land subsidence is increasing. So your costs are rising.\'94\
Deltas, after all, have been doing their thing for thousands upon thousands of years \'97 flooding, washing in sediment, rebuilding land. But we changed the system \'97\'a0meaning that simply building higher, on sinking land, may not be enough\'a0of a sustainable solution.\
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\cf0 www.washingtonpost.com/news/energy-environment/wp/2015/08/06/the-global-threat-to-river-deltas-and-the-people-who-live-on-them/}